Understanding verb tenses in English can be tricky, especially with irregular verbs like “cost.” Many learners assume the past tense should be formed by adding “-ed,” which leads to confusion.
The verb cost is commonly used when talking about prices, expenses, and value in everyday English. Using the wrong past tense can make sentences sound incorrect or unclear.
In this article, we’ll explain whether the past tense of cost is “cost” or “costed.” You’ll also see clear examples to help you use each form correctly and confidently.
Why This Simple Word Causes So Much Confusion
Let’s be honest — “cost” is one of those everyday English words that almost everyone uses without thinking. You say it at the store, in emails, during meetings. But the moment someone asks you to write its past tense, suddenly it feels like a trick question.
Is it cost? Is it costed? Or are both somehow acceptable? The answer is actually more interesting than you might expect — because both forms are correct, just in completely different situations. And once you understand why, you will never mix them up again.
English verbs generally fall into two camps when it comes to past tense. Regular verbs just get an -ed added at the end — you walked, you jumped, you talked. Clean and predictable.
Then there are irregular verbs — and these ones refuse to play by the rules. Words like cut, put, run, and cost don’t add -ed. They either stay the same or change into an entirely different form. And cost happens to be one of those verbs that stays exactly the same in the past tense.
But then people stumble across “costed” somewhere — maybe in a business report or a project budget — and suddenly everything feels uncertain. Is it a mistake? Is it informal? The truth is, it is neither. It is a deliberately different verb doing a very specific job.
When the Past Tense Is Simply “Cost”
For most everyday situations, the past tense of cost is just cost. No extra letters. No change whatsoever. If you are talking about how much something actually cost you — money that has already left your pocket — then cost is your word.
Some Quick Examples
“The flight to Istanbul cost me $620.”
“That old laptop cost her nearly $1,200 three years ago.”
“His reckless decision cost the team an entire contract.”
See the pattern? In every single one of these sentences, we are talking about something that has already happened — a price already paid, a consequence already felt. No guesswork. No estimation. Just a fact.
When “Costed” Is the Correct Choice
Here is where things get genuinely interesting. “Costed” is a real, legitimate word — but it means something subtly different from cost. While cost tells you about a price that was actually paid, costed describes the process of figuring out what something will cost.
Think of it this way: cost = the bill you already paid. costed = the math you did before paying anything. You will find costed used constantly in business, finance, project management, and accounting.
Professional Examples
“The engineering team costed the bridge renovation at $4.2 million.”
“Before submitting the proposal, we costed every single phase of the project.”
“The new software migration has been costed and is ready for board approval.”
In all of these, costed means “calculated the cost of” or “put a price estimate on.” Nobody has actually paid anything yet. That is the key difference.
Scenario 2 – Sending a Budget Estimate to a Stakeholder
Now imagine you are a project manager and you have just finished estimating the budget for a website redesign. Here is how you might communicate that:
Subject: Website Redesign – Budget Breakdown
Hi Rachel,
Our team has now costed the full website redesign project. The total estimate comes in at $195,000, which covers UI/UX design, front-end and back-end development, quality testing, and a 3-month support window after launch.
I have attached a detailed cost breakdown for your review. Happy to walk through it on a call.
Best, Nathan
Notice that Nathan uses “costed” — not “cost” — because his team went through the process of estimating and calculating. The money has not been spent yet. It is still a projection. That is exactly the situation where costed belongs.
Cost vs. Costed – A Clear Comparison
| Situation | Which Form? | Example |
|---|---|---|
| Money was already spent | cost | The trip cost $900. |
| A price was estimated or calculated | costed | We costed the event at $15,000. |
| Something caused a loss or consequence | cost | The delay cost us two clients. |
| A team did a budgeting or pricing exercise | costed | They costed the project last week. |
| Casual, everyday writing | cost | Lunch cost me $18. |
| Formal business or financial writing | costed | The expansion was costed at $3M. |
The Three Most Common Mistakes – And How to Fix Them
Even people who write professionally every day get this wrong sometimes. Here are the exact mistakes to watch out for:
Mistake 1 – Using “Costed” When Talking About an Actual Purchase
✗ Wrong: The new office chairs costed the company $8,000.
✓ Correct: The new office chairs cost the company $8,000.
The chairs were bought. The money is gone. This is not an estimation — it is a completed transaction. So cost is the only correct choice here.
Mistake 2 – Using “Cost” When Describing a Budgeting Process
✗ Wrong: We cost the entire renovation last Tuesday.
✓ Correct: We costed the entire renovation last Tuesday.
The team sat down and calculated prices. They performed a costing exercise. Using just “cost” here makes the sentence sound grammatically broken — like something is missing. Costed completes the meaning.
Mistake 3 – Mixing Up Both Forms in the Same Sentence
This one actually trips up a lot of people — but it is not always a mistake. Sometimes both forms are correct in a single sentence, each doing a different job:
“We costed the kitchen renovation carefully, but in the end it cost us more than we expected.”
Read it again. “Costed” = the estimation phase. “Cost” = what actually ended up being spent. Both are right. Both are needed. The sentence would fall apart without either one.
Scenario 3 – A Casual Social Media Post
Not every example has to be a formal business email. Here is how both forms might appear naturally in everyday conversation:
“Finally got around to planning that kitchen makeover. We costed it out last month and yeah — it’s not cheap. But my neighbour did the same thing last year and it actually cost her way less than expected. So maybe there’s hope for us too.”
This sounds completely natural — because it is. “Costed it out” = the planning and estimating stage. “Cost her” = what her neighbour actually paid. Two different moments in time, two different verb forms, both fitting perfectly.
Scenario 4 – A Workplace Conversation
Here is a realistic back-and-forth between two colleagues after a team meeting:
“Hey, did you finish the estimate for the new warehouse?”
“Yeah, we costed it yesterday. Came in at around $2.1 million for the full build.”
“That’s actually not bad. My cousin built something similar last year and it cost him almost $2.8 million.”
“Yeah, prices have come down. We might actually stay under budget this time.”
Both speakers use the forms correctly without even thinking about it. “Costed it” = the team ran the numbers. “Cost him” = what the cousin actually paid in the past. In real conversation, this switch happens seamlessly.
A Simple Trick to Remember Which Form to Use
If grammar rules feel heavy, forget them for a second. Just ask yourself one question every time you are unsure:
“Is the money already gone — or am I still figuring out how much it will be?”
If the money is already spent → use cost.
If you are still estimating or calculating → use costed.
That is genuinely all there is to it. No complicated grammar terminology. No memorizing verb conjugation charts. Just one simple question, and the right answer follows every single time.
The Complete Verb Conjugation of “Cost”
| Verb Form | Example Sentence |
|---|---|
| Base form | How much does it cost? |
| 3rd person singular | It costs $60 a month. |
| Present participle | This project is costing more than planned. |
| Past tense | It cost $200 last week. |
| Past participle | It has cost us dearly over the years. |
Why Does English Have Both “Cost” and “Costed”?
A fair question. If cost already works as an irregular verb, why did English bother creating costed at all?
The answer is practicality. As businesses grew more complex, professionals needed a way to describe the act of calculating prices — not just the prices themselves. Saying “we cost the project” felt incomplete. It did not clearly communicate that a costing process had taken place.
So costed emerged naturally — a regular verb form built specifically to describe that activity. It is the same reason we say “I booked the flight” (made a reservation) rather than trying to stretch “book” into every possible meaning. Language evolves to be precise. And in modern business English, costed has firmly earned its place.
Final Thoughts
So what is the past tense of cost? For nearly every everyday situation, the answer is simply cost. It is an irregular verb. It does not change. It does not add -ed. It just stays as it is.
But if you work anywhere near finance, budgeting, or project planning, you will also come across costed — and it is not a typo or a grammatical error. It is a perfectly legitimate word that describes the specific process of estimating or calculating a cost.
The whole thing really comes down to this:
Already paid? → Cost.
Still calculating? → Costed.
Keep that in mind, and you will never second-guess yourself on this one again. English can be a confusing language — but not every part of it has to be.